Because there is quite evidently a gross disparity between the cost/benefit of having a stockpile of scrolls on hand.
Sure, wizards can get cheaper scrolls, but honestly, I don't see why a fireball scroll has to cost 300+ gp.
The price is not that high, but it prevents cheap and easy stockpiling of some of the more powerful scrolls.
Secondly and particularly for wizards; if you bring the scroll price closer to potion prices it means diminishing the perk of brew potion which is not a free feat like scribe scroll is.
Some spells (imo primarily offensive scrolls, except dispels) might be looked at individually but if it's all or nothing; better keep it this way.
Lowing the price of scribe scroll would be giving the wizards another powerful tool in their already stuffed belt.
Though I would love to have 10 firebrand scrolls for 200 gold each.
Pew Pew
Then maybe you should buy a wand instead.
Can't buy a wand of a level 5 spell.
Reducing lower level scrolls in cost probably wouldn't be so bad, but reducing the higher level ones is just asking for trouble.
Nigh invincible clerics with a stack of Heal scrolls doesn't sound too appealing really.